1. Field of the Invention
The present invention relates, for example, to a high-frequency differential amplifying system used for a mobile communications terminal.
2. Background Art
Recently, high performance of mobile communications terminals, such as cellular phones, necessitates increase in the electrical power consumption of parts other than the high-frequency parts, and a longer duration of call is required. Therefore, there has been an increasing demand, for example, for decreasing electrical power consumption in a high-frequency differential amplifying system used for a transmission part of a mobile communications terminal.
Generally, a differential amplifying system handling high frequency and a large amount of power is made up of a bias circuit and a differential amplifier and a load. In particular, in a differential amplifying system requiring low-distortion characteristics, bias voltage to be supplied to the differential amplifier for performing power amplification is fixed, and the bias voltage is determined so as to ensure linearity when signal amplitude is large. Accordingly, there has been a problem that large electrical power is consumed even when the signal amplitude is small, and thus that low electrical power consumption cannot be achieved.
Some conventional differential amplifying system intending to reduce electrical power consumption use a configuration, for example, in which an input power detection circuit consisting of a detector circuit and a low-pass filter is connected to the midst of a two-state amplifier circuit having a first-stage amplifier circuit and a final-stage amplifier circuit, through a series circuit consisting of a capacitor and a resistor, and in which the output of the input power detection circuit is compared in a comparator with a reference voltage and the resultant signal is used for two-stage variation of the bias current of the final-stage amplifier (e.g., Japanese Patent Laid-Open No. 2003-324323).
This differential amplifying system serves for reducing consumption of current in the final-stage amplifier circuit by changing the bias current.
However, in the conventional differential amplifying system mentioned above, if distortion is desired to be equal throughout a wide output signal range, for example, a plurality of bias circuits are required to be provided, which may raise a problem of making the circuit configuration complicated. In addition, at the timing when the bias voltage is outputted from each of the bias circuits, the bias current is drastically changed to deteriorate the distortion characteristics, which may raise a problem of significantly reducing efficiency when the bias current is changed.